Application of oxide semiconductors to thin-film transistors and transparent electrodes has attracted attention in recent years. Thin-film transistors that utilize oxide semiconductors are being actively applied to flat panel display devices like liquid crystal display devices and organic EL display devices. Moreover, transparent electrodes that utilize oxide semiconductors are being actively applied to flat panel display devices and touch panels.
In these fields of application, materials of low resistance and high electrical conductivity are used in the interconnections and electrodes of the oxide semiconductor.
In these fields of application, aluminum (Al) or Al alloy, molybdenum, and the like are used.
For example, an interconnection material composed of Ti/Al—Si/Ti has been proposed that has a structure of Al—Si sandwiched with Ti.
Copper has come to attract attention as a material having lower electrical resistance than these electrode materials. However, in addition to being poor in adhesion to the TFT substrate of an LCD, that is glass, copper also has a problem of being easily oxidized during insulation layer formation.
In recent years, therefore, attempts have been made to resolve such problems by using a technology that utilizes alloyed copper interconnections in TFT-LCD. This technology is aimed at ensuring adhesion to the substrate by virtue of the alloying elements forming a reaction product with the substrate, and simultaneously at the Cu oxidation resistance action of additive elements by forming oxide on the Cu surface.
However, the sought characteristics are not adequately achieved by the proposed technology. Specifically, the electrical resistance of the Cu increases due to alloying elements persisting in the Cu, and superiority to interconnection material using Al or Al alloy is not exhibited.
In addition, as set out in Patent document 1, a technology for using copper interconnections in TFT-LCD has been developed that forms a Mo alloy film between the Cu and substrate, thereby ensuring substrate adhesion and barrier performance.
However, this technology has problems in that a Mo alloy film-forming process is added and the effective resistance of the interconnections increases. In addition, Cu-only layers are used at the source electrode and drain electrode, so that an issue of their stability remains.
Further, Patent document 2 proposes a technology for overcoming the problems related to Cu interconnections by forming a high-melting-point nitride such as TaN, TiN, WN or the like around the Cu. However, as compared to a conventional interconnection material, this technology has problems in that a material for forming a barrier layer and an additional process are required, and that the effective resistance of the interconnections rises because the high-resistance barrier layer is formed to thick film thickness.
Further, Patent document 3 discloses improvement of adhesion and oxidation resistance by adding one or more elements among Mg, Ti and Cr to the Cu of the TFT-LCD interconnections. However, increase in interconnection resistance owing to the presence of residual additive elements in the interconnections is a problem. Another problem is that the additive elements reduce substrate oxides, and the reduced elements disperse in the interconnections to increase the interconnection resistance.
Patent document 4 discloses enhancement of oxidation resistance by adding 0.3 to 10 wt % Ag to Cu. However, problems exist in that adhesion to glass substrate is not improved and oxidation resistance sufficient to withstand the liquid crystal process is not obtained.
Patent document 5 proposes a copper alloy for improving adhesion obtained by adding 0.5 to 5 wt % of at least one element among Ti, Mo, Ni, Al and Ag to Cu. However, a problem exists in that the additive elements increase the electrical resistance of the interconnections.
Patent document 6 proposes adding 0.1 to 3.0 wt % Mo to Cu and segregating the Mo at the grain boundaries to inhibit oxidation by grain boundary diffusion. However, while this technology can improve Cu resistance to oxidation, it has a problem in that it increases interconnection resistance.
Patent document 7 uses a copper alloy obtained by adding a suitable additive element to Cu, whereby the additive element forms an oxide film acting as a protective coating that inhibits Cu oxidation and forms an insulating layer at an interface adjacent to the protective coating to inhibit interdiffusion. This provides copper interconnections high in electrical conductivity and excellent in substrate adhesion. A liquid crystal display device using the copper interconnections is also provided. It is suggested that one of these external additive elements is preferably Mn. However, this technology is not adequate for concretely realizing the special features of the interconnection structure and TFT electrode structure used in the liquid crystal display device.
Patent document 8 proposes a TFT structure used in a TFT-LCD, and for the case of applying the Cu alloy to a gate electrode, concretely presents a TFT structure whose gate electrode is coated with an oxide film. In this context, where the first metal is Cu, the second metal is designated as at least one selected from among Ti, Zr, Hf, Ta, Nb, Si, B, La, Nd, Sm, Eu, Gd, Dy, Y, Yb, Ce, Mg, Th and Cr. However, these oxide films have a problem in not being capable of adequately inhibiting interdiffusion with the insulating layer.
Non-patent document applies a copper electrode to a TFT using an oxide semiconductor. a-InGaZnOx is used for the oxide semiconductor, and a laminated structure of pure copper (Cu) and a copper alloy (CuMn) is used as the copper electrode. By this a TFT having about 10 times the mobility of the current a-Si TFT was realized to make high-speed operation possible. In addition, the copper electrode consisting of the aforesaid laminated structure was used to minimize interconnection resistance and increase the possibility of realizing higher definition in a flat display. However, further simplification of the electrode structure is required.